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De-excitation gamma-ray lines, produced by nuclei colliding with protons, provide information about astrophysical environments where particles have kinetic energies of 10–100 MeV per nucleon. In general, such environments can be categorized into two types: the interaction between non-thermal MeV cosmic rays and ambient gas, and the other is thermal plasma with a temperature above a few MeV. In this paper, we focus on the latter type and investigate the production of de-excitation gamma-ray lines in very hot thermal plasma, especially the dependence of the line profile on the plasma temperature. We have calculated the line profile of prompt gamma rays from 12C and 16O and found that when nuclei have a higher temperature than protons, gamma-ray line profiles can have a complex shape unique to each nucleus species. This is caused by anisotropic gamma-ray emission in the nucleus rest frame. We propose that the spectroscopy of nuclear de-excitation gamma-ray lines may enable to probe energy distribution in very hot astrophysical plasmas. This diagnostics can be a new and powerful technique to investigate the physical state of a two-temperature accretion flows onto a black hole, especially the energy distributions of the protons and nuclei, which are difficult to access for any other diagnostics. [ABSTRACT FROM AUTHOR]

MeV nuclear de-excitation lines serve as a unique tool to study low-energy cosmic rays (CRs), containing both spectral and elemental information of the interacting material. In this paper, we estimated the possible nuclear de-excitation lines from the young supernova remnant Cassiopeia A. Given different CR spectral shapes and interacting materials, we found the predicted fluxes of strong narrow line emissions from the remnant are highly model-dependent, ranging from about |$1\times 10^{-10}\, {\rm \, cm^{-2}\, s^{-1}}$| to |$1\times 10^{-6}\, {\rm \, cm^{-2}\, s^{-1}}$| for the 4.44 MeV narrow line and from about |$4\times 10^{-11}\, {\rm \, cm^{-2}\, s^{-1}}$| to |$2\times 10^{-7}{\rm \, cm^{-2}\, s^{-1}}$| for the 6.13 MeV narrow line, respectively. Based on the new estimation, we also discussed the detection probability of these line emissions against the MeV diffuse Galactic background under different assumptions of instrument response functions. [ABSTRACT FROM AUTHOR]

The active galactic nucleus 4C  + 28.07 is a flat-spectrum radio quasar, one of the brightest at γ-ray energies. We study its multiwavelength emission by analysing ∼12.3 yr of Fermi-LAT data in the γ-ray band and Swift-X-Ray Telescope (XRT)/Ultraviolet/Optical Telescope (UVOT) available data in X-ray and optical-to-ultraviolet bands. In the γ-ray band, five flaring periods have been detected, and quasi-simultaneously with these flaring times, the X-ray and UVOT data detected by Swift-XRT/UVOT have also been analysed. In one of the brightest flare periods (Flare 5; observed on 2018 October 12), the γ-ray flux reached (6.7 ± 0.81) × 10−6 photon cm−2 s−1 (∼31 × higher than the mean flux over 12.3 yr) with detection significance of σ = 6.1. The estimated variability time (∼2 h) constrains the γ-ray emitting region size to ≤9 × 1014 cm, which is close to the black hole radius. The spectral energy distributions (SEDs) in the γ-ray band for the ∼12.3 yr of data show an early cut-off at ∼14 GeV; beyond ∼60 GeV, however, the spectrum hardens and is detected up to ∼316 GeV. Similar spectral behaviour is also noticeable for the SEDs of flares, which can be linked to the photon absorption by the emitting region's internal and external narrow-band radiation fields. In the quiescent period, the γ-ray emission was described by the synchrotron self-Compton scenario, while the external photons contributions from the disc and the broad-line region were required to explain the short-term flaring γ-ray emission. Considering the significance of the obtained results from 4C  + 28.07, we compared the parameters with 3C 279 and M87, to motivate further studies. [ABSTRACT FROM AUTHOR]

Star-forming regions have been proposed as potential Galactic cosmic ray accelerators for decades. Cosmic ray acceleration can be probed through observations of gamma-rays produced in inelastic proton–proton collisions at GeV and TeV energies. In this paper, we analyse more than 11 yr of Fermi –LAT data from the direction of Westerlund 2, one of the most massive and best-studied star-forming regions in our Galaxy. In particular, we investigate the characteristics of the bright pulsar PSR J1023–5746 that dominates the gamma-ray emission below a few GeV at the position of Westerlund 2 and the underlying extended source FGES J1023.3–5747. The analysis results in a clear identification of FGES J1023.3–5747 as the GeV counterpart of the TeV source HESS J1023-575, through its morphological and spectral properties. This identification provides new clues about the origin of the HESS J1023-575 gamma-ray emission, favouring a hadronic origin of the emission, powered by Westerlund 2, rather than a leptonic origin related to either the pulsar wind nebula associated with PSR J1023–5746 or the cluster itself. This result indirectly supports the hypothesis that star-forming regions can contribute to the cosmic ray sea observed in our Galaxy. [ABSTRACT FROM AUTHOR]

X-ray observations of kiloparsec-scale jets indicate that a synchrotron origin of the sustained non-thermal emission is likely. This requires distributed acceleration of electrons up to near PeV energies along the jet. The underlying acceleration mechanism is still unclear. Shear acceleration is a promising candidate, as velocity-shear stratification is a natural consequence of the collimated flow of a jet. We study the details of shear acceleration by solving the steady-state Fokker–Planck-type equation and provide a simple general solution for trans-relativistic jets for a range of magnetohydrodynamic turbulent power-law spectra. In general, the accelerated particle population is a power-law spectrum with an exponential-like cut-off, where the power-law index is determined by the turbulence spectrum and the balance of escape and acceleration of particles. Adopting a simple linearly decreasing velocity profile in the boundary of large-scale jets, we find that the multiwavelength spectral energy distribution of X-ray jets, such as Centaurus A and 3C 273, can be reproduced with electrons that are accelerated up to ∼PeV. In kpc-scale jets, protons may be accelerated up to ∼EeV, supporting the hypothesis that large-scale jets are strong candidates for ultra-high-energy-cosmic ray sources within the framework of shear acceleration. [ABSTRACT FROM AUTHOR]

In nuclei of starburst galaxies (SBGs), the combination of an enhanced rate of supernova explosions and a high gas density suggests that cosmic rays (CRs) can be efficiently produced, and that most of them lose their energy before escaping these regions, resulting in a large flux of secondary products, including neutrinos. Although the flux inferred from an individual starburst region is expected to be well below the sensitivity of current neutrino telescopes, such sources may provide a substantial contribution to the diffuse neutrino flux measured by IceCube. Here, we compute the gamma-ray and neutrino flux due to SBGs based on a physical model of CR transport in a starburst nucleus, and accounting for the redshift evolution of the number density of starburst sources as inferred from recent measurements of the star formation rate. The model accounts for gamma-ray absorption both inside the sources and in the intergalactic medium. The latter process is responsible for electromagnetic cascades, which also contribute to the diffuse gamma-ray background at lower energies. The conditions for acceleration of CR protons up to energies exceeding |$\sim 10 \, \rm PeV$| in starburst regions, necessary for the production of PeV neutrinos, are investigated in a critical way. We show that starburst nuclei can account for the diffuse neutrino flux above |$\sim 200 \, \rm TeV$|⁠ , thereby producing |$\lesssim 40 {\rm { per\, cent}}$| of the extragalactic diffuse gamma-ray background. Below |$\sim 200 \, \rm TeV$|⁠ , the flux from starburst appears to be somewhat lower than the observed one, where both the Galactic contribution and the flux of atmospheric neutrinos may account for the difference. [ABSTRACT FROM AUTHOR]

The Crab Nebula is an extreme particle accelerator that boosts the energy of electrons up to a few PeV (⁠|$10^{15} \ \rm eV$|⁠), close to the maximum energy allowed theoretically. The physical conditions in the acceleration site and the nature of the acceleration process itself remain highly uncertain. The key information about the highest-energy accelerated particles is contained in the synchrotron and inverse Compton (IC) channels of radiation at energies above 1 MeV and 100 TeV, respectively. A recent report of the detection of an ultra-high-energy gamma-ray signal from the Crab Nebula up to 300 TeV allows us to determine the energy distribution of the highest-energy electrons and to derive the magnetic field strength in the acceleration region, |$B\le 120\rm \, \mu G$|⁠ , in a parameter-free way. This estimate brings new constraints on the properties of non-thermal particle distributions and places important constraints on the magnetohydrodynamic models for the Crab Nebula, in particular on the feasible magnetization and anisotropy of the pulsar wind. The calculations of synchrotron and IC emission show that future observations with instruments that allow detection of the Crab Nebula above 300 TeV and above 1 MeV will clarify the conditions that allow acceleration of electrons beyond PeV energies in the Crab Nebula. In particular, we will be able to verify the hypothetical multicomponent composition of the electron energy distribution, and we will determine the magnetic field strength in the regions responsible for the acceleration of PeV electrons. [ABSTRACT FROM AUTHOR]

Sources of X-rays such as active galactic nuclei and X-ray binaries are often variable by orders of magnitude in luminosity over time-scales of years. During and after these flares the surrounding gas is out of chemical and thermal equilibrium. We introduce a new implementation of X-ray radiative transfer coupled to a time-dependent chemical network for use in 3D magnetohydrodynamical simulations. A static fractal molecular cloud is irradiated with X-rays of different intensity, and the chemical and thermal evolution of the cloud are studied. For a simulated |$10^5\, \mathrm{M}_\odot$| fractal cloud, an X-ray flux <0.01 erg cm−2 s−1 allows the cloud to remain molecular, whereas most of the CO and H2 are destroyed for a flux of ≥1 erg cm−2 s−1. The effects of an X-ray flare, which suddenly increases the X-ray flux by 105×, are then studied. A cloud exposed to a bright flare has 99 per cent of its CO destroyed in 10–20 yr, whereas it takes >103 yr for 99 per cent of the H2 to be destroyed. CO is primarily destroyed by locally generated far-UV emission from collisions between non-thermal electrons and H2; He+ only becomes an important destruction agent when the CO abundance is already very small. After the flare is over, CO re-forms and approaches its equilibrium abundance after 103–105 yr. This implies that molecular clouds close to Sgr A⋆ in the Galactic Centre may still be out of chemical equilibrium, and we predict the existence of clouds near flaring X-ray sources in which CO has been mostly destroyed but H is fully molecular. [ABSTRACT FROM AUTHOR]

The energy spectra of TeV gamma-rays from blazars, after being corrected for intergalatic absorption in the extragalactic background light (EBL), appear unusually hard, a fact that poses challenges to the conventional models of particle acceleration in TeV blazars and/or to the EBL models. In this paper, we show that the internal absorption of gamma-rays caused by interactions with dense narrow-band radiation fields in the vicinity of compact gamma-ray production regions can lead to the formation of gamma-ray spectra of an almost arbitrary hardness. This allows significant relaxation of the current tight constraints on particle acceleration and radiation models, although at the expense of enhanced requirements to the available non-thermal energy budget. The latter, however, is not a critical issue, as long as it can be largely compensated by the Doppler boosting, assuming large (>10) Doppler factors of the relativistically moving gamma-ray production regions. The suggested scenario of formation of hard gamma-ray spectra predicts detectable synchrotron radiation of secondary electron–positron pairs which might require a revision of the current ‘standard paradigm’ of spectral energy distributions of gamma-ray blazars. If the primary gamma-rays are of hadronic origin related to pp or interactions, the ‘internal gamma-ray absorption’ model predicts neutrino fluxes close to the detection threshold of the next generation high-energy neutrino detectors. [ABSTRACT FROM AUTHOR]

We provide a systematic numerical and analytical study of Klein–Nishina (KN) effects in the spectrum produced by a steady-state, non-thermal source where rapidly accelerated electrons cool by emitting synchrotron radiation and Compton up-scattering ambient photons produced outside the source. We focus on the case where q, the ratio of the ambient radiation field to source magnetic field energy densities, significantly exceeds unity. We show that the KN reduction in the electron Compton cooling rate causes the steady-state electron spectrum to harden at energies , where and is the characteristic ambient photon energy. This hardening becomes noticeable in the synchrotron radiation from electrons with energies as low as and changes the synchrotron spectral index relative to its Thomson limit value by as much as for . The source synchrotron spectrum thus shows a high-energy ‘bump’ or excess, even though the electron acceleration spectrum has no such excess. In contrast, the low-energy Compton gamma-ray spectrum shows little distortion because the electron hardening compensates for the KN decline in the scattering rate. For sufficiently high electron energies, however, Compton cooling becomes so inefficient that synchrotron cooling dominates – an effect omitted in most previous studies. The hardening of the electron distribution thus stops, leading to a rapid decline in Compton gamma-ray emission, i.e. a strong spectral break whose location does not depend on the maximum electron energy. This break can limit the importance of Compton gamma-ray pair production on ambient photons and implies that a source's synchrotron luminosity may exceed its Compton luminosity even though . We discuss the importance of these KN effects in blazars, micro-quasars and pulsar binaries. [ABSTRACT FROM AUTHOR]

We present the results from the Hitomi Soft Gamma-ray Detector (SGD) observation of the Crab nebula. The main part of SGD is a Compton camera, which in addition to being a spectrometer, is capable of measuring polarization of gamma-ray photons. The Crab nebula is one of the brightest X-ray/gamma-ray sources on the sky, and the only source from which polarized X-ray photons have been detected. SGD observed the Crab nebula during the initial test observation phase of Hitomi. We performed data analysis of the SGD observation, SGD background estimation, and SGD Monte Carlo simulations, and successfully detected polarized gamma-ray emission from the Crab nebula with only about 5 ks exposure time. The obtained polarization fraction of the phase-integrated Crab emission (sum of pulsar and nebula emissions) is (22.1% ± 10.6%), and the polarization angle is |${110{^{\circ}_{.}}7}$| + |${13{^{\circ}_{.}}2}$| /− |${13{^{\circ}_{.}}0}$| in the energy range of 60–160 keV (the errors correspond to the 1 σ deviation). The confidence level of the polarization detection was 99.3%. The polarization angle measured by SGD is about one sigma deviation with the projected spin axis of the pulsar, |${124{^{\circ}_{.}}0}$|  ±  |${0{^{\circ}_{.}}1}$|⁠. [ABSTRACT FROM AUTHOR]

The article presents a correction to the article "Klein–Nishina Effects in the Spectra of Non-Thermal Sources Immersed in External Radiation Fields," published in the previous issue of the journal "Monthly Notices of the Royal Astronomical Society."